Horizontal drilling systems currently in use include technology known as mud motor technology, pipe-in-pipe technology, rotary steerable devices, and hammer technology. Each system has inherent limitations related to the system's operation and method of use.
Mud motor technology utilizes drilling fluid to transfer power from a drill rig located at a ground surface, through a drill string comprised of inter-connected drill rods, to a down-hole motor. The drill string is connected to the rear end of the mud motor is connected; while a drill bit, attached to an output shaft, is connected to a front end of the mud motor. The drill bit is powered rotationally by torque generated by drilling fluid passing through the motor. The drill bit can thus be rotated, while the drill string is held from rotating. Directional control is achieved by the addition of an offset coupling that offsets the center-line of the drill bit from the center-line of the drill string and mud motor. In particular, to control the direction of the drill bit, the drill string is held from rotating, and the drill bit rotated by the mud motor. The drill sting then moves the assembly longitudinally forward, creating a bored hole in the direction of the centerline of the drill bit. To bore a straight hole, the drill string, mud motor and offset coupling are all rotated to create a bored hole in the direction of the centerline of the drill string.
One limitation of mud motors is related to the capacity to transmit power to the drill bit. Since the drill string is not rotationally secured to the drill bit, the mud motor must provide the rotational power to the bit. The length of the motor is typically a function of the rotational power provided to the bit. In some applications, the length required to develop sufficient torque is significant. Further, the construction of mud motors is such that they are typically less flexible than the drill rod. This combination of length and stiffness can limit the directional control capability of mud motor systems.
A second inherent limitation of mud motors is related to the use of the drilling fluid to provide rotational power to the drill bit. Since mud flow rate and pressure determine the power transferred to the drill bit, the rate and pressure must be maintained in order to maintain drilling speed. In some situations, other aspects of drilling are affected by the flow rate of the drilling mud, and it may be desirable to reduce either the flow rate or the pressure. These situations compromise the efficiencies of the contrasting aspects of a drilling operation. For instance, a “frac-out” can occur as a result of excessive flow or excessive pressure of the drilling fluid. A frac-out situation is where drilling fluid is forced though a fracture in the ground rather than through the bored hole. In a frac-out situation, it is desirable to reduce flow rate or fluid pressure to cease further expansion of the ground fracture. Preferably, the flow rate and pressure are at an initially reduced level to prevent the probability of a frac-out altogether. However, reducing the flow rate and pressure negatively affects drilling performance.
A third inherent limitation is related to the need for the drill bit to be offset from the centerline of the mud motor. This offset requires a complicated drive shaft assembly in order to transfer the rotary power through the offset. The drill bit is mounted to the drive shaft, which is inherently more flexible than the motor housing. The resulting assembly has several limitations including significant initial cost associated with the complicated assembly, limited durability, and a flexibility that can affect the dynamic stability of the drill bit during drilling.
Pipe-in-pipe technology operates in a similar fashion. The drill bit is oriented at an end of an outer drill string with a center that is offset with respect to the center of the outer drill string. An inner pipe rotationally powers the drill bit independent from rotation of the outer drill string. To achieve directional control of the drill bit, the outer drill string is held from rotating while the inner drill pipe rotates the drill bit. The drill string is then moved forward to create a bored hole in the direction of the offset. To bore a straight hole, the outer drill string, the inner drip pipe and the drill bit are all rotated to create a bored hole in the direction of the centerline of the outer drill string.
One limitation of this technology relates to the size of the component that provides rotational power to the drill bit, i.e., the inner pipe. Because the diameter of the inner pipe is smaller that the outer drill string, the maximum torque that can be transferred to the drill bit is less than the maximum torque that could be transferred by the outer drill string.
A second limitation of pipe-in-pipe technology is related to the inherent flow restriction of the pipe-in-pipe configuration. Drilling fluid is required to cool the drill bit and to transfer the cuttings out of the bored hole. The rate of drilling can be limited by the fluid flow rate. The cross-sectional area of the inner drill pipe, which is used to transfer the fluid, is less than the cross-sectional area of the outer drill string. Thus, the maximum flow rate is lower, or the fluid pressure at the drill rig is higher for a given flow rate, with a pipe-in-pipe system as compared to other systems utilizing the outer drill string for fluid transfer.
Rotary steerable devices include a down-hole housing mounted on the drill string on bearings such that the housing can remain stationary while the drill string rotates. A drill bit is powered rotationally by an extension of the drill string and a drive shaft that extends through the down-hole housing. The down-hole housing has some form of offset to subject the drill bit to an unbalanced load condition, causing it to change the direction of the borehole. The orientation of the down-hole housing determines the boring direction of drill bit.
A limitation of rotary steerable devices is related to the fact that there is a non-fixed relationship between the down-hole housing and the drill string. Many designs have been developed to control of the position of the housing relative to the drill string. Typically the designs involve manipulating the drill string. Any change in orientation of the down-hole housing in relation to the drill string during general operation will affect the direction of the bored hole. Changes in orientation of the housing relative to the drill string are unpredictable making operation complicated and the results unreliable.
Hammer technology utilizes drilling fluid to transfer power from the drill rig at the surface, through a drill string comprised of inter-connected drill rods, to a down-hole hammer. The drill string is connected to a rear end of the hammer. A drill bit, attached to an output shaft of the hammer, is connected at a front-end of the hammer. The drill bit is powered longitudinally with impact impulses from the hammer. The drill bit is able to cut through hard materials such as rock, without requiring full rotation of the drill bit. To achieve directional control, the drill string is oscillated rather than rotated. For example, the drill string can be oscillated slightly while the drill bit is cutting with the impact impulses generated by the fluid activated hammer to control the direction of boring. Specifically, the drill bit is oriented in manner such that an effective center of the bit is offset from the center of the drill string while the drill string is moved forward. To bore a straight hole, the drill string, the hammer, and the drill bit are all rotated to create a bored hole in the direction of the centerline of the drill string.
A limitation of the hammer technology is related to the capability of the drilling fluid, used with currently available hammers, to carry cuttings. Commercially available hammers useful for this type of horizontal boring are activated with compressed air. The capability of compressed air to carry and transport sizable cuttings is less than the capability of drill mud used with either mud motors or pipe-in-pipe technology. Further, the maximum length of a bored hole is limited by the capability of the fluid to transfer the cuttings a particular distance.
Thus, a need exists for a versatile drilling tool that reduces the effect of the above noted limitations.